As a cathode active material contained in a cathode of a lithium ion secondary battery (hereinafter a lithium ion secondary battery will sometimes be referred to as LIB), a composite oxide of Li and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2 or LiMnO4 has been used.
A LIB is required to maintain a discharge capacity and a discharge voltage even after repeating the charge and discharge cycle (hereinafter referred to as cycle characteristics), and to have a high energy per unit volume of the cathode (hereinafter referred to as an energy density). However, a conventional cathode active material cannot sufficiently meet such requirements. For example, a LIB using LiCoO2 as a cathode active material can have a high initial energy density by discharging at a high voltage of 4.5 V. However, if it is charged at a high voltage, its cycle characteristics are low, and if the charge and discharge cycle is repeated, the discharge capacity and the discharge voltage are lowered. As a result, if the charge and discharge cycle is repeated, the energy density of the LIB is also extremely lowered.
As a cathode active material with which a LIB having favorable cycle characteristics even when charged at a high voltage is obtained, the following material has been proposed.
(1) A cathode active material comprising a composite oxide containing Li and at least one transition metal element selected from the group consisting of Ni, Co and Mn, and an oxide of at least one metal element selected from the group consisting of Zr, Ti and Al, and at least one carbon material selected from the group consisting of carbon nanotubes, graphene and carbon black present on the surface of the composite oxide (Patent Document 1).
However, in the case of a LIB using a cathode active material in which a metal oxide and a carbon material are present on the surface of a composite oxide, if the proportion of the cathode active material in the cathode is increased (for example, at least 90 mass %) so as to increase the energy density, the cycle characteristics are lowered. Accordingly, the proportion of the cathode active material in the cathode may not be increased and as a result, the energy density may not sufficiently be increased.